The two-cycle engine of a motor-bicycle is generally provided with a crank case within which is defined a transmission chamber within which, in turn, a transmission mechanism and a gear change mechanism are accommodated.
With the crank case of the type described above, the transmission chamber is defined separately from a pre-loading chamber at a position behind the crank case. A transmission mechanism comprising a counter shaft and a drive shaft is arranged within the upper portion of the transmission chamber and a gear change mechanism comprising first and second shift fork shafts, a gear shift cam shaft located between the first and second shift fork shaft, and a gear shift shaft for operating the gear shift cam shaft is located within the lower portion of the transmission chamber.
With the crank case of the construction described above, substantially all of the whole constructional elements or members constituting the transmission mechanism and the gear change mechanism are accommodated within the transmission chamber. This means, in other words, that the transmission chamber occupies a relatively large space within the crank case, which results in the lowering of the rigidity of the crank case. The lowering of the rigidity of the crank case, in turn, results in an increase in the transfer of the vibrations from the engine itself.
In the meantime, the lubrication of the two-cycle engine of the motor-bicycle is generally carried out by means of a lubricating oil, that is, a transmission oil, stored within a clutch chamber and which is scattered by means of the rotation of a primary gear of a clutch into the transmission chamber by means of the so-called splashing lubrication method. According to such splashing lubrication method, however, it takes a considerable amount of time to increase the upper or surface level of the lubricating oil which has been scattered throughout the transmission chamber such that the level reaches a predetermined position, in the case where the transmission chamber has a large inner volume. Accordingly, in an adverse case, there may be caused a seizure of the variable speed gear of the gear change mechanism and the like due to oil starvation. This is a significant problem for the operation of the engine of the motor-bicycle.